The present invention relates to the method of reducing emissions of nitrogen oxides and, in particular, relates to a method of reducing nitrogen oxide emissions resulting from burning nitrogen containing fuels.
In recent years, there has been a growing concern over the problem of air pollution. This problem has become acute in industrialized urban areas of the country. There are a variety of sources of air pollution such as the internal combustion engine, chemical and metallurgical plants, power generating plants, etc. One of the more serious pollutants is the nitrogen oxides such as NO and NO.sub.2 (hereinafter referred to collectively as "NO.sub.x "). The nitrogen oxides contribute to air pollution by the formation of photochemical smog.
A source of NO.sub.x emissions is fuel burning plants such as power generating plants, incinerators, etc. In fuel burning plants, there are two sources of NO.sub.x emissions. The first source of NO.sub.x emissions originates from the thermal fixation of atmospheric nitrogen at the elevated temperatures obtained during the combustion process. The second source of NO.sub.x emissions originates from the thermal conversion of some of the organically-bound nitrogen in the hydrocarbon fuel to NO.sub.x during the combustion process. In most cases, depending upon the combustion technique, about 15 to about 30% of the organically-bound nitrogen is converted to NO.sub.x. Unfortunately, commercial methods of denitrification consume relatively large amounts of hydrogen and are thus an expensive and inefficient method of removing organically-bound nitrogen from hydrocarbon fuel. In several areas where air quality control regulations have been promulgated, inexpensive high nitrogen containing fuels cannot be burned in fuel burning plants. This is a substantial problem because there exists a shortage of inexpensive low nitrogen containing fuels. Thus, there is a significant need for a method to reduce NO.sub.x emissions from the combustion of high nitrogen fuels in fuel burning plants.
One prior method of reducing NO.sub.x emissions from fuel burning plants comprises blending fuels containing a small amount of organically-bound nitrogen with fuels containing larger amounts of organically-bound nitrogen to obtain a fuel mixture having a more acceptable amount of nitrogen. However, this method requires the use of substantially greater amounts of low nitrogen containing fuels than high nitrogen containing fuels to obtain a mixture having an acceptable level of nitrogen. Alternatively, this method requires the consumption of large amounts of hydrogen in commercial denitrification processes to reduce the nitrogen content of the fuel at a relatively high refining cost.
Another prior method of reducing NO.sub.x emissions from fuel burning plants comprises off-stoichiometric combustion of the fuel. This type of combustion was accomplished in a furnace having two sets of burners which were vertically spaced apart. Fuel-rich combustion was carried on in the lower burners and air-rich combustion was simultaneously carried on in the upper burners.
In fuel-rich combustion, the oxygen selectively reacts with the hydrocarbon fuel due to the oxygen deficient atmosphere, thereby reducing the flame temperature and the amount of thermal fixation of atmospheric nitrogen. The fuel-rich combustion also results in the formation of relatively stable reduced nitrogen species. The formation of these more stable nitrogen species minimizes the conversion of organically-bound nitrogen in the fuel into NO.sub.x. Unfortunately, the fuel-rich combustion also causes thermal cracking of the unburned fuel, thereby resulting in the formation of a significant amount of condensable carbon or smoke. To avoid giving off smoke, the upper burners were operated in an air-rich manner. The air-rich combustion functioned to completely burn any unburned fuel in the combustion gases from the fuel-rich combustion. However, the excess amount of oxygen provided to the upper burners resulted in increased conversion of the organically-bound nitrogen into NO.sub.x from the fuel supplied to the upper burners. Thus, a method which will produce a further reduction of NO.sub.x emissions is still required.